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Reports of resistance

There has been a report in west Tennessee, says Delaney, of fungicide-resistant frogeye leafspot. The fungus Cercospora sojina causes the disease, however, it can be seedborne. Frogeye leaf spot is most likely to become a problem if infected seed is planted or if the disease occurred in the previous year’s soybean crop and the land is not rotated. Extended periods of wet weather during the growing season will favor disease development.

“They’ve picked up some resistance in west Tennessee, in some fields in river bottoms. This particular area had been in soybeans for at least three years. It had been sprayed at least three years with a strobilurin fungicide. The producer noticed that after the first spray, there was a lot of frogeye leafspot. He sprayed again, and it kept coming. True resistance was confirmed in that field. As with glyphosate resistance, the same chemistry was used year after year,” he says.

In this case, the chemistry was strobilurin, which is the most effective tool currently available for controlling a broad sprectrum of soybean diseases.

“Just like weed resistance, the best way to insure we don’t have this problem in Alabama is to rotate crops, plant resistant varieties, and spray only when needed. We also can rotate our fungicides. Some of the triazole fungicides like Domark have some pretty good activity on frogeye leafspot. Stratego and Quardris Xtra are pre-mixes of a triazole and strobilurin and also could be options,” says Delaney.

Herbicide resistance management strategies for cotton also hold true for soybeans, he says.

There are fast-changing lineups in soybean varieties from year to year, and it can be difficult to keep up, says Delaney. “A variety may last three years at the most, and then it is replaced by a new, hopefully better, variety.”

He advises growers to use multiple sources of information when selecting varieties, being careful to choose them for the nematode and disease resistance required by a particular field situation.

New varieties are in the pipeline, he says. “Dicamba or 2,4-D-resistant soybeans are probably several years off in the future, maybe 2013 or 2014, and they’ll be very early maturing varieties in the beginning. It’ll probably be 2015 before we see any of those varieties well-suited for the South.”

Delaney also notes that iron chlorosis has been a problem in Alabama in fields with high-pH soils. “We looked at some in-furrow treatments last year of iron chelates, and some of it looked pretty good.”